The present invention is directed to a continuously variable transmission and, in addition, is also directed to an apparatus having a continuously variable transmission.
When choosing a power source for a particular application, the ideal choice would produce power with peak efficiency at exactly the torque and angular velocity required. Since this is impractical, a mechanical transmission is typically used with a power source to provide the appropriate torque and angular velocity required. Power is given by the following equation:
Power=Torque*Angular Velocity
In a mechanical transmission, a governing equation is:
PowerIn=PowerOut+Power Loss
Rewriting this equation in terms of torque and angular velocity provides:
TorqueIn*Angular VelocityIn=(TorqueOut*Angular VelocityOut)+Power Loss
In the case of an automobile, the required output conditions, angular velocity and torque, are always changing. Finding an engine capable of changing to match the output conditions is extremely difficult. An automobile typically uses an internal combustion engine as a power source while the driver controls the desired output condition. This application requires widely varied torque and angular velocities at the output depending on the driving condition. An internal combustion engine is simply not able to provide all of the varied torque and angular velocities while maintaining a high efficiency.
Vehicle designers overcame this limitation by using a manual transmission with several gear ratios. The vehicle driver manually changes the gear ratio to suit the driving environment. Around 1950, the automatic transmission was employed in vehicles to make the gear ratio changes automatically, but still incrementally. Neither the manual transmission nor the automatic transmission attempts to maintain the engine performance at an optimum level. Rather, these transmissions cause the engine to operate around its optimum level during each gear. This leads to an engine design that must have an extensive range of acceptable operating conditions. The engine is required to have reasonable power output over a fairly wide range of angular velocity conditions.
This requirement typically forces the engine to be less efficient than otherwise might be obtainable because an engine with a very narrow band of operation can more easily be optimized. It is conceivable to have an engine with peak power, torque, and efficiency all occurring at the same angular velocity. If these parameters decline sharply as the engine operates at points other than the optimum, the transmission would need many gear ratios to couple the engine to the desired output condition.
In order to use such an optimized engine, a continuously variable transmission (CVT) is employed. The CVT allows the engine to operate at its optimum while the vehicle operates at a widely varied set of driving conditions. To accomplish this, an infinite number of gear ratios are available between the input and output shafts of the transmission. By continuously varying from one gear ratio to another slightly different gear ratio, the CVT optimally couples the desired engine performance to the desired vehicle performance.
In optimizing a vehicle, it is realized that at certain vehicle operating conditions the peak power of an engine is greater than the power the vehicle can use. For example, a car at rest can be accelerated by the application of a torque; however, the total power that it can handle at zero velocity is zero. This is because power equals torque multiplied by angular velocity. When the angular velocity is zero, so is the power. An engine running at its optimal point would still produce power at zero angular velocity, leaving excess power. In a conventional car, such excess power is unacceptable because it cannot be used.
A hybrid vehicle, which is a cross between an electric car and a traditional car, makes efficient use of excess power by generating electricity from the engine power and storing this energy in a battery. Later, this stored energy can be used when required rather than asking for more power from the engine. An electric motor, powered by the vehicle battery, provides the propulsion force in a series hybrid vehicle. A parallel hybrid vehicle uses both the internal combustion engine as well as an electric motor to drive the vehicle. In a parallel hybrid, both power sources (the engine and the electric motor) have transmission paths to drive wheels of the vehicle.
A hybrid vehicle can operate at very high efficiency for several reasons. Electric motors typically have much higher efficiencies than an internal combustion engine, and electric motors typically have a broader range of efficient operating conditions. In addition, the series hybrid vehicle allows the engine to be set up to operate only at its peak performance point.
The design of parallel hybrid vehicles is somewhat limited by the transmission options. The engine still requires a multi-gear transmission to get the operation in the neighborhood of the vehicle requirements. At 20 miles per hour with the engine at peak output, the gear ratio is quite different than at 60 miles per hour with the engine at the same condition. A continuously variable transmission solves these gearing issues.
A CVT can be used with an impulse drive. However, an impulse drive is impractical for automotive applications due to the discontinuous output speed, which leads to pulsating power.
Several different concepts for a non-pulsating CVT have been designed. The most common is a belt-pulley system where the two pulleys are split axially into two halves. At least one half of each pulley can slide along the axis of rotation. A belt is set on the pulleys and by moving one pulley half closer to the stationary pulley half, the belt is forced away from the axis of rotation. The second pulley in the system does the exact opposite by moving the pulley half away from the stationary pulley half thereby causing the belt to move closer to the axis of rotation. The location of the belt on the driving pulley relative to the location on the driven pulley determines the gear ration. Visually, this is similar to a bicycle chain drive. As the chain moves to the larger sprocket at the pedals, the input to output gear ratio decreases. Moving the chain to the smaller sprocket at the wheel also causes the gear ratio to decrease. One disadvantage of the belt-pulley system is that there is a high friction load at the belt-pulley interface. The belt must have enough friction at the pulley to transmit the force required to move the vehicle; however, it also must have a low enough friction to slide easily up and down the faces of the pulley. These are contradictory requirements.
A variant on the belt-pulley concept has been designed which employs radial grooves on the two pulleys. The belt has sheaves that slide laterally within the belt to engage the grooves. This gives a positive drive and reduced radial sliding friction, but also generates friction inside the belt by sliding the sheaves back and forth.
Other known techniques employ balls or wheels with a tilting axis of rotation to couple a driving wheel to a driven wheel at varying points. Still other known techniques use a set of cones and a sliding ring or wheel or ball to couple the cone sets.
In all of the aforementioned non-pulsating CVT systems, a sliding element is employed within the power path of the transmission. This design tends to lead to contradictory requirements of adequate power transfer and minimal power to change gears. In addition, the sliding elements consume power as they are loaded and unloaded even when the gear ratios are not changing. Ideally, a CVT would continuously vary the gear ratio and require no power to do so. Also, an ideal CVT would not consume power just to maintain a constant operating gear ratio.
Thus, it is desirable to have a continuously variable transmission that couples the input power source to the output application without the use of sliding friction elements such as belts and wheels.
The present invention is a continuously variable transmission comprising first and second planetary gear sets. Each of the first and second planetary gear sets includes a sun gear member, a ring gear member, at least one planet gear meshing with the sun gear member and with the ring gear member, and a planet gear carrier member. The at least one planet gear is rotatably mounted to the planet gear carrier member. A first drive drivingly connects a first one of the members of the first planetary gear set and a first one of the members of the second planetary gear set. A second drive drivingly connects a second one of the members of the first planetary gear set and a second one of the members of the second planetary gear set. A third drive is drivingly connected with a third one of the members of the first planetary gear set. A fourth drive is drivingly connected with a third one of the members of the second planetary gear set.
In accordance with a preferred embodiment of the present invention, a first one of the drives is associated with a power input device, a second one of the drives is associated with a power output device, and a third one of the drives is associated with an additional power device. The first and second planetary gear sets define a torque loop for transmitting power from at least one of the power input device and the additional power device to the power output device.
The present invention also provides an apparatus comprising first and second planetary gear sets. Each of the first and second planetary gear sets includes a sun gear member, a ring gear member, at least one planet gear meshing with the sun gear member and with the ring gear member, and a planet gear carrier member. The at least one planet gear is rotatably mounted to the planet gear carrier member. A first drive drivingly connects a first one of the members of the first planetary gear set and a first one of the members of the second planetary gear set. A second drive drivingly connects a second one of the members of the first planetary gear set and a second one of the members of the second planetary gear set. A third drive is drivingly connected with a third one of the members of the first planetary gear set. A fourth drive is drivingly connected with a third one of the members of the second planetary gear set. A power input device is associated with a first one of the drives. A power output device is associated with a second one of the drives. At least one additional power device is associated with a third one of the drives.
In accordance with one embodiment of the invention, the power input device comprises an internal combustion engine, the power output device comprises a driven device, and the additional power device comprises an electric machine, whereby the first and second planetary gear sets define a continuously variable transmission for a hybrid vehicle. The electric machine comprises an electric motor/generator which is capable of either supplying or removing power from the continuously variable transmission.